Microporous thermoplastic membranes can be used as battery separators in, e.g., primary and secondary lithium secondary batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous thermoplastic membranes are used for battery separators, particularly lithium ion battery separators, the membranes' properties significantly affect the performance, productivity and safety of the resulting batteries. Such microporous polymeric (e.g., thermoplastic) membranes can be produced, e.g., by “dry” or “wet” processes. Dry processes involve producing the membrane from a polymer melt. Wet processes involve combining one or more polymers with diluent (e.g., solvent) to form a polyolefin solution, extruding the polyolefin solution to form a sheet, and then removing at least a portion of the diluent form the sheet to form a polymeric membrane. As might be appreciated, diluent removal is an important process step in the wet process.
In a wet process, the diluent (also called a solvent, membrane-forming solvent, or process solvent) is generally removed from a cooled extrudate to form the polymeric membrane. In the wet process, a solvent washing step (also called a solvent treatment) can be used to remove solvent from the extrudate. Generally, the diluent used to produce the polymer solution is called the “process solvent” or “P-Sol”, and the washing solvent used for washing (or displacing) the process solvent from the extrudate is called the “washing solvent”. For example, JP60-242035A discloses that a chlorine containing solvent, such as dichloromethane (DCM) can be used as a washing solvent to remove a liquid paraffin process solvent from an extrudate. One problem with this approach is that the residual DCM in pores of microporous membrane after washing can degrade the membrane's properties. While it might be possible to overcome this problem by, e.g., adding an agent capable of modifying the surface tension of the residual DCM in the membrane's pores, residual surface agent in the membrane's pores would might undesirable affect the membrane's porosity.
JP2002-012694 A, JP2002-012695A and JP2002-256099 A disclose washing solvents such as hydrofluoroether (FIFE), either alone or in combination with other solvents such as decane. While HFE has a low surface tension, and HFE does not degrade microporous membrane's properties as much as DCM alone, HFE is not as good a washing solvent as DCM because HFE is less miscible with commonly-used process solvents (such as liquid paraffin) than is DCM. Consequently, the rate of removing process solvent by washing the extrudate with HFE is lower than when DCM is used. Since the removal rate is lower, longer washing times are needed, which causes a decrease in the amount of membrane produced by the wet process. Mixtures of HFE and DCM have been proposed for process solvent removal since HFE-DCM mixtures generally have a lower surface tension than DCM. Consequently, using an HFE-DCM mixture as a washing solvent does not degrade microporous membrane's properties as much as when the washing solvent is DCM alone. Moreover, mixtures of DCM and HFE are more miscible with commonly used process solvents than is HFE alone.
JP2002-256099 A discloses using DCM as a first washing solvent in a first washing stage and HFE as a second washing solvent in a second washing stage downstream of the first washing stage. The second washing stage uses HFE to rinse away at least some of the DCM remaining in the extrudate after the first washing stage. For this reason, the FIFE can be called a rinsing solvent. Using this approach, the problems associated with residual DCM in the membrane's pores can be overcome since the HFE rinsing step removes the undesirable DCM. Even so, such a process is difficult to operate continuously because the DCM rinsed from the membrane in the second stage will accumulate in the FIFE. Accordingly, the FIFE can be replaced with fresh HFE, and/or the DCM should be separated from the FIFE so that the HFE can be made available for recycle and re-use. The first option is inefficient and expensive. The second option (removing the DCM from the HFE and recycling the purified HFE) is problematic because FIFE is miscible with DCM. Moreover, HFE and DCM are difficult to separate by conventional methods (e.g., fractional distillation) because they have similar boiling points. Finally, even if separation by fractional distillation could be accomplished efficiently, HFE and DCM together form an azeotrope which contains a significant amount of both HFE and DCM. As might be appreciated, such an azeotrope cannot be separated by distillation.
It would therefore be desirable to further improve the wet process by efficiently recycling and re-using DCM, FIFE, or both.